Mainly because of the higher energy of blue light than red light.
In fact, light is made of photons, each one carrying an energy equal to
[tex]E=hf[/tex]
where h is the Planck constant while f is the frequency of the light.
The frequency of red light is approximately 450 THz, while the frequency of blue light is about 650 Hz. Higher frequency means higher energy, so blue light is more energetic than red light and therefore it can cause more damages than red light.
